scholarly journals Reaction of Chromium(III) with 3,4-Dihydroxybenzoic Acid: Kinetics and Mechanism in Weak Acidic Aqueous Solutions

2008 ◽  
Vol 2008 ◽  
pp. 1-8 ◽  
Author(s):  
Kimon Zavitsanos ◽  
Konstantinos Tampouris ◽  
Athinoula L. Petrou
Keyword(s):  
Aqueous Solutions ◽  
Coordination Sphere ◽  
Activation Parameters ◽  
Experimental Results ◽  
Ligand Concentration ◽  
Water Molecules ◽  
Dihydroxybenzoic Acid ◽  
Proton Release ◽  
Three Stages ◽  
Ph Decrease

The interactions between chromium(III) and 3,4-dihydroxybenzoic acid (3,4-DHBA) were studied resulting in the formation of oxygen-bonded complexes upon substitution of water molecules in the chromium(III) coordination sphere. The experimental results show that the reaction takes place in at least three stages, involving various intermediates. The first stage was found to be linearly dependent on ligand concentrationk1(obs)_=k0+k1(obs)[3,4-DHBA], and the corresponding activation parameters were calculated as follows:ΔH1(obs)≠=51.2±11.5 kJ mol−1,ΔS1(obs)≠=−97.3±28.9 J mol−1 K−1(composite activation parameters) . The second and third stages, which are kinetically indistinguishable, do not depend on the concentrations of ligand and chromium(III), accounting for isomerization and chelation processes, respectively. The corresponding activation parameters areΔH2(obs)≠=44.5±5.0 kJ mol−1,ΔS2(obs)≠=−175.8±70.3 J mol−1 K−1. The observed stages are proposed to proceed via interchange dissociative (Id, first stage) and associative (second and third stages) mechanisms. The reactions are accompanied by proton release, as is shown by the pH decrease.

scholarly journals Kinetics and Mechanisms of the Chromium(III) Reactions with 2,4- and 2,5-Dihydroxybenzoic Acids in Weak Acidic Aqueous Solutions

2010 ◽  
Vol 2010 ◽  
pp. 1-10 ◽  
Author(s):  
Kimon Zavitsanos ◽  
Athinoula L. Petrou
Keyword(s):  
Hydrogen Bonding ◽  
Aqueous Solutions ◽  
Activation Parameters ◽  
Intramolecular Hydrogen Bonding ◽  
Dihydroxybenzoic Acid ◽  
Proton Release ◽  
Second Stage ◽  
Ph Decrease ◽  
Intramolecular Hydrogen ◽  
Two Stages

The reactions of 2,4- and 2,5-dihydroxybenzoic acids (dihydroxybenzoic acid, DHBA) with chromium(III) in weak acidic aqueous solutions have been shown to take place in at least two stages. The first stage of the reactions has an observed rate constantk1(obs)=k1[DHBA]+Cand the corresponding activation parameters areΔH1(2,4)≠=49,5 kJ/mol−1,ΔS1(2,4)≠=−103,7J mol−1K−1,ΔH1(2,5)≠=60,3 kJ/mol−1, andΔS1(2,5)≠=−68,0 J mol−1K−1. These are composite activation parameters and the breaking of the strong intramolecular hydrogen bonding in the two ligands is suggested to be the first step of the (composite) first stage of the reactions. The second stage is ligand concentration independent and is thus attributed to a chelation process. The corresponding activation parameters areΔH2(2,4)≠=45,13 kJ/mol−1,ΔS2(2,4)≠=−185,9 J mol−1K−1,ΔH2(2,5)≠=54,55 kJ/mol−1, andΔS2(2,5)≠=−154,8 J mol−1K−1. The activation parameters support an associative mechanism for the second stage of the reactions. The various substitution processes are accompanied by proton release, resulting in pH decrease.

scholarly journals Kinetics and Mechanism of the Reaction between Chromium(III) and 2,3-Dihydroxybenzoic Acid in Weak Acidic Aqueous Solutions

2010 ◽  
Vol 2010 ◽  
pp. 1-8 ◽  
Author(s):  
Athinoula L. Petrou ◽  
Vladimiros Thoma ◽  
Konstantinos Tampouris
Keyword(s):  
Aqueous Solutions ◽  
Activation Parameters ◽  
Entropy Change ◽  
Hydroxyl Group ◽  
Positive Entropy ◽  
Dihydroxybenzoic Acid ◽  
Three Stages ◽  
Ph Decrease ◽  
Two Stages ◽  
Mechanism Of The Reaction

The reaction between chromium(III) and 2,3-dihydroxybenzoic acid (2,3-DHBA) takes place in at least three stages, involving various intermediates. The ligand (2,3-DHBA)-to-chromium(III) ratio in the final product of the reaction is 1 : 1. The first stage is suggested to be the reaction of[Cr(H2O)5(OH)]2+with the ligand in weak acidic aqueous solutions that follows anIdmechanism. The second and third stages do not depend on the concentrations of chromium(III), and their activation parameters areΔH≠=61.2±3.1 kJmol−1,ΔS≠=−91.1±11.0 JK−1mol−1,ΔH≠=124.5±8.7 kJmol−1, andΔS≠=95.1±29.0 JK−1mol−1. These two stages are proposed to proceed via associative mechanisms. The positive value ofΔS≠can be explained by the opening of a four-membered ring (positive entropy change) and the breaking of a hydrogen bond (positive entropy change) at the associative step of the replacement of the carboxyl group by the hydroxyl group at the chromium(III) center (negative entropy change in associative mechanisms). The reactions are accompanied by proton release, as shown by the pH decrease.

scholarly journals Kinetics and Mechanism of the Reaction between Chromium(III) and 3,4-Dihydroxy-Phenyl-Propenoic Acid (Caffeic Acid) in Weak Acidic Aqueous Solutions

2008 ◽  
Vol 2008 ◽  
pp. 1-7 ◽  
Author(s):  
Vladimiros Thoma ◽  
Konstantinos Tampouris ◽  
Athinoula L. Petrou
Keyword(s):  
Aqueous Solutions ◽  
Metal Ion ◽  
Activation Parameters ◽  
Ligand Molecule ◽  
Propenoic Acid ◽  
Protonation Equilibrium ◽  
Three Stages ◽  
Ph Decrease ◽  
Rapid Protonation ◽  
Mechanism Of The Reaction

Our study of the complexation of 3,4-dihydroxy-phenyl-propenoic acid by chromium(III) could give information on the way that this metal ion is available to plants. The reaction between chromium(III) and 3,4-dihydroxy-phenyl-propenoic acid in weak acidic aqueous solutions has been shown to take place by at least three stages. The first stage corresponds to substitution (Idmechanism) of water molecule from the Cr(H2O)5OH2+coordination sphere by a ligand molecule. A very rapid protonation equilibrium, which follows, favors the aqua species. The second and the third stages are chromium(III) and ligand concentration independent and are attributed to isomerisation and chelation processes. The corresponding activation parameters areΔH2(obs)≠= 28.6±2.9 kJmol−1,ΔS2(obs)≠=−220  ±10 JK−1mol−1,ΔH3(obs)≠= 62.9±6.7 kJmol−1andΔS3(obs)≠=−121  ±22 JK−1mol−1. The kinetic results suggest associative mechanisms for the two steps. The associatively activated substitution processes are accompanied by proton release causing pH decrease.

Long-Term Correlations In Diffusive Motion Of Water Molecules And Rare Gas Atoms In Helium And Argon Aqueous Solutions

2015 ◽  
Vol 60 (8) ◽  
pp. 757-763 ◽  
Author(s):  
V.P. Voloshin ◽  
◽  
G.G. Malenkov ◽  
Yu.I. Naberukhin ◽  
◽  
...  
Keyword(s):  
Aqueous Solutions ◽  
Rare Gas ◽  
Water Molecules ◽  
Diffusive Motion ◽  
Rare Gas Atoms

Konkurrierende Liganden: Theophyllin als nicht- und stark koordinierender Ligand in Quecksilber(II)-Komplexen / Competing Ligands: Theophylline as Non- and Strongly Coordinating Ligand in Mercury(II) Complexes

2006 ◽  
Vol 61 (6) ◽  
pp. 758-765 ◽  
Author(s):  
Matthias Nolte ◽  
Ingo Pantenburg ◽  
Gerd Meyer
Keyword(s):  
Oxygen Atom ◽  
Water Molecule ◽  
Aqueous Solutions ◽  
Single Crystal ◽  
Coordination Polymers ◽  
Water Molecules ◽  
Slow Evaporation ◽  
Monoclinic Symmetry ◽  
X Ray ◽  
Chloride Ligand

[{Hg(CF3)2}(ThpH)(H2O)](H2O) (1), [{Hg4(Thp)4}(ClO4)4(H2O)8](H2O)4 (2), [{Hg(ThpH)2} (NO3)](NO3) (3) and {Hg(Thp)Cl}(H2O) (4) (ThpH = theophylline, C7H8N4O2) have been synthesized by slow evaporation of aqueous solutions of the mercuric salts Hg(CF3)2, Hg(ClO4)2, Hg(NO3)2, or HgCl2 and theophylline. Their crystal structures were determined on the basis of single crystal X-ray data. The coordination polymers 1 and 2 crystallize with triclinic symmetry, P1̅ (no. 2), with a = 468.8(2), b = 1256.4(5), c = 1445.5(6) pm, α = 67.15(3), β = 89.21(3), γ = 89.40(3)° and a = 833.6(1), b = 1862.7(2), c = 2182.9(2) pm, α = 111.61(1), β = 90.98(1), γ = 95.51(1)°, respectively. 3 and 4 crystallize with monoclinic symmetry, Pc (no. 7), a =1194.1(1), b=1258.8(2), c=735.5(2) pm, β =96.96(2)° and P21/n (no. 14), a=1069.0(2), b =911.6(1), c=1089.9(2) pm and β = 96.87(2)°. In 1 the theophylline molecules are non-coordinating to mercury and leave the Hg(CF3)2 molecule unchanged. Only weak electrostatic attractions to one keto-oxygen atom of theophylline and one water molecule hold this co-crystallisate together. In 2, the theophyllinate anion, Thp−, strongly coordinates with both N(7) and N(9) to HgII forming a large ring with eight Hg atoms that incorporates the water molecules. One sort of nitrate ions in 3 is weakly attached to HgII with the theophylline molecules still bound strongly through N(9). The chloride ligand and the theophyllinate ion seem to have the same strengths as ligands in 4 as they are both attached to HgII with the shortest distances possible

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